Some weeks ago I bought a chinese amplifier kit on Ebay which came for 16 Euro / Dollar shipped. The kit took only 2,5 weeks to arrive and came in a simple plastic bag without any protection and it also came without a manual or instructions. But even if it does not work, the parts alone are worth the money. While waiting for the kit I found an interesing article from John, PD7MAA on his website. He does not only have the schematic available, he also shows how nice such DIY-projects can look like.
Especially the filter is worth a look. Building the kit was not too hard, but the surface-mount (SMD) parts are very tiny and you need a calm hand and good eyes. When I had everything assembled I tested it without a heatsink on the two IRF530N mosfets from IOR. It took not more than five seconds to kill both FETs while sending an AM carrier (1,5W drive). I ordered a pack of five new IOR IRF530N mosfets on Ebay which arrived today. After the QRL I soldered them in, and I drilled the holes into two pieces of an old heatsink harvested from an old PC power supply.
I did some power output and SWR measurements then. Numbers indicates the output and the numbers in brackets are "SWR-bars" on the FT-817 display:
|0,5W||60 (0)||55 (0)||40 (0)||28 (1)||20 (0)||20 (1)||13 (0)||10 (0)||8 (0)|
|1,0W||71 (0)||70 (0)||58 (1)||48 (2)||30 (0)||25 (3)||20 (0)||18 (0)||14 (0)|
|2,5W||89 (0)||79 (2)||72 (4)||58 (3)||60 (2)||52 (3)||58 (1)||60 (3)||40 (0)|
|5,0W||95 (1)||81 (3)||75 (4)||60 (4)||74 (3)||60 (4)||72 (0)||62 (3)||60 (0)|
After the "measurements" I was able to work S57DX on 20m with 60W output (2,5W drive from the FT-817) and I also worked my friend Peter, DC2ZO. While in the QSO I pushed 5W from the 817 into the amp and with 3 SWR-bars on the dispay and 74W output I killed another pair of IRF530N mosfets. I will now mount a bigger heatsink, 2 fans and a case and then I will do some more measurements. As recommended by John, PD7MAA a drive of 2,5W seems to be ideal for me too.
Anton, HB9ASB already wrote an article about the 70W kit: FT-817 PA - Bausatz ohne Schema
Youtube-video from KC2IRV: